Many satellite and terrestrial optical communication systems require transmission of analog signals. A straightforward way to address this need is to modulate the amplitude (AM) of an optical carrier. This approach, however, suffers from a poor Signal to Noise Ratio (SNR) at a distant receiver. It is well known that broadband modulation schemes, which utilize higher bandwidth than that of the transmitted waveform, may improve the SNR over that achieved with AM. Pulse position modulation (PPM) is one of such techniques. In PPM, a shift in the pulse position represents a sample of the transmitted waveform, as shown in FIG. 1. It can be shown that for a given power, SNRPPM∝SNRAM(tp/t)2, where tp is the spacing between un-modulated pulses and t is the pulse duration, respectively. See H. S. Black, “Modulation Theory”, D. Van Nostrand Co. (1953).
The implementations of PPM for optical communications require new techniques for generating trains of optical pulses whose positions are shifted in proportion to the amplitude of a transmitted waveform. A bandwidth of Δf=1-10 GHz and higher is of interest for future communications. Since pulse repetition frequencies (PRF) of 1/tp>2Δf are required for sampling a signal of bandwidth Δf, GHz trains of picosecond (ps) pulses may be required for realizing the advantages of PPM. For example, a free space optical link designed to transmit waveforms with Δf=10 GHz bandwidth requires sampling rates of PRF=1/tp≧2Δf=20 GHz. By employing 1-2 ps-long optical pulses, a 30 dB gain is realized over an AM system with equal optical power.
Optical PPM offers large SNR improvements in power-starved optical links. This technology, however, requires development of new types of optical PPM receivers. The simplest and most basic PPM decoder, which is based on an integrating circuit, suffers from poor performance at low frequencies. See H. S. Black, “Modulation Theory”, noted above. Though newly invented PPM decoders overcome the low-frequency shortcomings of the simplest decoder, these improvements come at the cost of higher complexity. See, for example, S. I. Ionov, “Detection of optical analog PPM streams based on coherent optical correlation”, U.S. Pat. No. 6,462,860; S. I. Ionov, “A practical design of a PPM receiver with optical top hat pulse generator controlled by solitons”, U.S. patent application Ser. No. 10/341,689 filed Jan. 13, 2003 which is based upon U.S. Ser. No. 60/383,343 filed May 23, 2002; I. Ionov “Method and Apparatus for PPM Demodulation Using A Semiconductor Optical Amplifier”, U.S. Pat. No. 7,330,304 filed Nov. 3, 2003; I. Ionov, “Method and apparatus for optical top-hat pulse generation”, U.S. patent application Ser. No. 10/735,071 filed Dec. 12, 2003 which is based upon U.S. Ser. No. 60/488,540 filed Jul. 18, 2003; and S. I. Ionov, “Interferometric PPM Demodulators based on Semiconductor Optical Amplifiers”, U.S. Pat. No. 7,149,029 filed Jan. 11, 2005.
In the past, ElectroOptic delay generators shift the temporal position of an optical pulse in proportion to the applied voltage. Such a PPM modulator provides seamless means for a PPM encoding scheme wherein a temporal displacement of an optical pulse from its unmodulated position represents a sample of the transmitted waveform. Such an ElectroOptic delay generator has been described in: Method and apparatus for Electro-optic Delay Generation of Optical Signals U.S. Pat. No. 6,466,703 filed Apr. 7, 2000;
More recently, U.S. Pat. No. 7,330,304 demonstrated a PPM decoder based on the gain dynamics of a semiconductor optical amplifier. When fed by two optical streams—a PPM signal and clock, the decoder produces an electric output that is proportional to the delays between the corresponding signal and clock pulses and changes on the pulse-by-pulse scale.
A PPM communication system based on such an encoder and decoder requires optical clock pulses, which must be either transmitted alongside with the PPM signal or regenerated at the receiver side. This requirement puts an unnecessary burden on the communication system, which requirement is eliminated according to the present disclosure.